Drive system having at least one hydraulic actuator

ABSTRACT

The invention relates to a drive system having at least one hydraulic actuator ( 2 ), which is supplied with pressure medium ( 4 ) by means of at least one pressure-medium pump ( 3 ) in a main circuit, the pressure-medium pump ( 3 ) being a rotational-speed-controlled pressure-medium pump that is driven by a motor ( 5 ) at a variable rotational speed and a controlled torque, said drive system being characterized in that leakage fluid ( 7 ) that escapes from a displacement chamber ( 6 ) of the pressure-medium pump ( 3 ) is discharged through a fluid-conducting connection ( 8 ) having at least one check valve ( 9, 16 ), and the at least one check valve ( 9, 16 ) is installed in the fluid-conducting connection ( 8 ) in such a way that the check valve prevents pressure medium from flowing back out of the main circuit into the displacement chamber ( 6 ) or another interior chamber of the pressure-medium pump ( 3 ) and feeds leakage fluid ( 7 ) into the main circuit of the drive system ( 1 ) when there is a slight excess pressure.

The invention relates to a drive system having at least one hydraulic actuator, which is supplied with a pressure medium by means of at least one pressure medium pump in a main circuit, wherein the pressure medium pump is a speed-controlled pressure medium pump, which is driven by a motor at a variable speed and a controlled torque.

Drive systems of a hydraulic nature comprising one or more actuators, which can also execute different operating movements in a sequential manner, are often supplied with hydraulic energy by means of a control valve circuit that is driven by one or more pressure medium pumps at a constant speed. The technical drawback of such drive systems is the poor efficiency, because the control valves convert hydraulic energy into thermal energy in accordance with the inherent characteristic of the operating principle.

Such drive systems are also known in suspension systems for motor vehicles. Hence, DE 199 30 444 C2 describes an active stabilizer for a motor vehicle that couples two wheels of a vehicle axle. This stabilizer consists of a first stabilizer part, which is assigned to one wheel, and a second stabilizer part, which is assigned to the other wheel, as well as an actuator, which couples the stabilizer parts. The actuator controls the lateral inclination of the motor vehicle by bracing the stabilizer parts against each other. Each stabilizer part extends from one port assigned to the actuator, to a port that is assigned to the wheel. In the meantime, there also exist active vehicle suspension systems, which are further developed to form complex combination systems and enable not only an active roll stabilization but also a level control of a top section of the chassis. In this case, the term level control is understood to mean the active change in the distance between the top section of the chassis and a vehicle axle.

As a rule, two-axle passenger vehicles may have a level control system at both the front axle and the rear axle. In this case, the level control at the vehicle axles can be carried out independently of each other.

DE 10 2004 039 973 A1 describes an active vehicle suspension system comprising a level control. Such a suspension system exhibits good spring properties and reduces the vehicle's tendency to tilt. At the same time, this suspension system makes it possible to raise and lower the top section of the chassis relative to the wheels of the vehicle. This lifting and lowering is achieved by means of the simultaneous possible actuation of a directional control valve for changing the pressure in the piston-side cylinder chamber of the active vehicle suspension system in connection with an adjustable throttle valve and a vehicle part which enables an adjusting action and is disposed between a piston rod-side cylinder chamber of the suspension system and a pressure medium pump.

A drive system of the type described in the introductory part is known from the prior art US 2008/0190104 A1. The prior art drive system comprises an actuator with a reversible motor, which is designed to generate a variable torque. Furthermore, this drive system comprises a hydraulic transducer, which is connected to the reversible motor and has a first port and a second port. The pressure chambers provided in a hydraulic cylinder can be charged by way of the ports.

Based on this prior art, the object of the present invention is to provide a hydraulic drive system which is designed in a simple way, has good control properties, and avoids the throttling losses of the control valves that are inherent in the operating principle, so that very good efficiency is achieved, as required.

This object is achieved with a drive system having the features specified in claim 1 in its entirety.

According to the characterizing part of claim 1, the leakage fluid, escaping from a displacement chamber of the pressure medium pump, is discharged through a fluid-conducting connection having at least one check valve, wherein the at least one check valve is installed in the fluid-conducting connection in such a way that the check valve prevents the pressure medium from flowing back out of the main circuit into the displacement chamber or another interior chamber of the pressure medium pump and feeds the leakage fluid into the main circuit of the drive system when there is a slight excess pressure.

In order to compensate for the leakage losses in the hydraulic circuit between the hydraulic actuator and the pressure medium pump, the invention provides that the leakage fluid, escaping from the pressure medium pump, is discharged through a check valve by means of a fluid-conducting connection to the pressure medium flow generated by said connection. The use of a check valve makes it possible to actuate, on the one hand, the leakage fluid flow; and, on the other hand, the leakage oil in the entire pressure medium flow can be kept in circulation with any other quantity of fluid.

Furthermore, it is advantageous to discharge the leakage fluid at a low pressure port of the pressure medium pump or to feed the leakage fluid to the low pressure side of such a pressure medium flow of the pressure medium pump. Furthermore, it is advantageous to feed the leakage fluid flow to a high pressure port of the pressure medium pump. In this case, the check valve or the two check valves, which prestress the leakage fluid flow as a function of the direction of flow of the pressure medium pump, prevent the pressure medium from flowing, subject to a high pressure, into the corresponding interior chamber of the housing of the pressure medium pump. In this respect, it is provided that the check valves have mutually opposite opening directions. Each of the fluid-conducting connections that discharge the leakage fluid is connected to a high pressure port of the pressure medium pump, so that the overall result is a parallel connection of the fluid-conducting connecting lines to the high pressure fluid lines to the consumers or the hydraulic actuators that are supplied by the pressure medium pump. The leakage is recirculated, as required, through the respective low pressure fluid line, which is designed in the form of the high pressure fluid line and, at this point, is not needed for the high pressure guide.

The pressure medium pump can drive a whole variety of different kinds of hydraulic actuators, such as a double-acting actuating cylinder or a hydraulic motor, which can also be used to actuate stabilizers of a roll stabilization system of a vehicle.

Due to the fact that the pressure medium pump, which supplies the hydraulic actuator with a pressure medium, is operated preferably as a bi-directional speed-controlled pump, preferably without a control valve and, thus, with extremely low losses, in a two or four-quadrant operating mode, and is driven by a motor with variable speed and controlled torque, the result is a drive system that delivers pressure medium from a pressure medium container to the respective hydraulic actuator and vice versa, only as required and in such a way that the pressure medium is adapted to the desired actuation.

Furthermore, it is advantageous to charge a pressure accumulator with a pressure medium; and this pressure accumulator can be used for compensating a volume of the pressure medium on the consumer side or also on the intake side of the pressure medium pump. The pressure accumulator can also be used to directly charge a working chamber of a hydraulic actuator. If the intake side of the pressure medium pump is charged from the pressure accumulator, then cavitation in the drive system is avoided by means of this measure of prestressing the system.

The pressure medium pump has a constant displacement volume, so that in combination with the torque-controlled, speed-variable drive of the pressure medium pump, variable flow rates are made possible. As a result, there is no need for valves to control the flow rate and the direction of flow of the pressure medium pump. Check valves, which have an impact on the leakage fluid outflow, are adequate to operate the drive system. However, it may be advantageous to connect fast switching directional control valves between the pressure medium pump and the hydraulic actuator or between a plurality of hydraulic actuators.

If the hydraulic actuator is, for example, a double-acting cylinder, then both ports of the pressure medium pump are connected in each instance to a working chamber of the pressure medium pump, so that in the event of a rotational direction or direction of flow of the pressure medium pump in the one direction, for example, the rod chamber is filled and the working chamber on the piston base side is emptied and vice versa. The result of this feature is an exact control and travel movement of the actuating element of the hydraulic actuator as a function of the direction of rotation and the rotational speed of the pressure medium pump. The motor, preferably an electric motor, which is driving the pressure medium pump, is torque-controlled and driven as a function of the sensor signals of a control and/or regulating device, which determines the demand requirement of the hydraulic actuator. In this respect, the motor can be, for example, an electric motor with pulse width modulation.

In order to superimpose a level control function on the functions of the roll stabilization, it is advantageous to assign a valve to each hydraulic actuator that is associated with the drive system. This valve can be used to empty or fill a working chamber of the respective hydraulic actuator, so that, for example, the absolute position of a piston of an actuating cylinder can be displaced.

As a result, the absolute position of the wheels of a motor vehicle relative to a chassis can be changed in terms of their height; and the top section of the chassis of a motor vehicle can be raised or lowered.

Hydraulic actuators, for example, in the manner of actuating cylinders, can also be coupled so that the result is a sequence control or a coupling in the opposite direction. For example, a vehicle wheel on the inside of a curve can be raised; and the vehicle wheel that is on the outside of a curve can be lowered.

Additional advantages, features, and details of the invention will be apparent from the dependent claims and the following description, in which a number of exemplary embodiments are described with reference to the drawings. In this context, the features mentioned in the claims and the description may be essential to the invention individually or in any combination.

FIG. 1 is a schematic circuit diagram of a drive system comprising a double-acting actuating cylinder and a pressure accumulator as the consumer;

FIG. 2 is a schematic circuit diagram of a drive system comprising a double-acting actuating cylinder and a pressure accumulator, which charges a working chamber of the actuating cylinder;

FIG. 3 shows a drive system, the consumer of which is an oscillating motor for a roll stabilization system, with a pressure accumulator for compensating for the leakage fluid and for prestressing the pressure medium on the intake side;

FIG. 4 shows a drive system, as in FIG. 3, but without a pressure accumulator;

FIG. 5 shows a drive system, the consumers of which are two actuating cylinders with a cross-connected working chamber, and a pressure accumulator for compensating for the leakage fluid and for prestressing the pressure medium on the intake side;

FIG. 6 shows a drive system, which corresponds to the one in FIG. 5, but without a pressure accumulator; and

FIG. 7 is a schematic circuit diagram of a drive system with two actuating cylinders as the consumers in conformity with the drawing according to FIG. 6, where the supply of pressure medium to the actuating cylinders can be overridden by one 2/2-way valve respectively in order to remove the pressure medium from each working chamber respectively of each actuating cylinder.

FIG. 1 shows a schematic circuit diagram of a hydraulic drive system 1 comprising a hydraulic actuator 2, which is designed as a double-acting actuating cylinder 19. The hydraulic actuator 2 is provided, as is generally known, with a piston 20, which is guided in a cylinder housing 21 in such a way that it can be displaced in the axial direction; and this hydraulic actuator forms two working chambers 13, 13′ in the cylinder housing 21. A pressure medium pump 3, which is provided with a constant displacement volume, is used to supply pressure medium to the actuator 2. The pressure medium pump 3 is driven by a motor 5, constructed in the form of a torque-controlled, speed-variable electric motor, by means of a shaft 22, which is merely indicated symbolically. However, the electric motor 5 can also be replaced with a conventional internal combustion engine, for example, in the form of a diesel engine. Furthermore, it is possible to use an axial piston machine (not illustrated), which is the subject matter of DE 10 2007 058 859 A1, where the electric motor and the pressure medium pump are combined as a module in a common housing.

In the exemplary embodiment according to FIG. 1, the two working chambers 13, 13′ of the actuating cylinder 19 are connected in a fluid-conducting manner to the high pressure port 15 of the pressure medium pump 3; and in the embodiment according to FIG. 2, each working chamber 13, 13′ of the actuating cylinder 19 is connected to a high pressure port 15 or 15′ respectively of the pressure medium pump 3. In this respect, the pressure medium 4 is conveyed, as a function of the direction of rotation of the motor 5 and the rotational speed, into the one or the other working chamber 13, 13′ of the actuating cylinder 19 by the pressure medium pump 3 in a closed hydraulic circuit without the interconnection of switching valves or directional control valves. The result is a travel movement or, if desired, a blocking of the position of the piston 20 of the respective actuating cylinder 19. A four-quadrant drive of the motor 5 or more specifically a four-quadrant operation of the pressure medium pump 3 is provided, a feature that clearly demonstrates a use of the drive system 1 in a motor vehicle (not illustrated) as an active stabilization system.

A pressure accumulator 10 serves to compensate for the fluid with respect to the differential volume between the two chamber sides of the actuating cylinder 19 or the pressure medium pump 3 as well as to prestress the pressure medium 4 on the high pressure side of the pressure medium pump 3. In the drive system 1 shown in FIG. 1, the two working chambers 13, 13′ are charged to the same degree with the pressure of the pressure accumulator 10, whereas in the exemplary embodiment of a drive system 1 shown in FIG. 2, the piston rod-side working chamber 13′ of the actuating cylinder 19 is directly charged with the pressure of the pressure accumulator 10 without the interconnection of the pressure medium pump 3. The pressure medium pump 3 has a low pressure port 14 in both of the possible directions of flow. The low pressure port is effective whenever the respectively other, opposite high pressure port 15, 15′ makes the high pressure that is required for the hydraulic circuit available. The respective low pressure port 14 produces a port for a fluid-conducting connection 8, which returns again the leakage fluid 7 from a displacement chamber 6 of the pressure medium pump 3, and, in particular, over the low pressure port to the respective high pressure side and, as required, to the high pressure ports 15, 15′. This approach represents a design feature for an overall hydraulically closed drive system 1 without a pressure medium container.

The fluid-conducting connection 8 runs through the respective low pressure port 14 to each high pressure port 15, 15′; and each connecting branch of the connection 8 has a check valve 9, 16. Each check valve 9, 16 is installed in the fluid-conducting connection 8 in such a way that the check valve prevents the pressure medium from flowing back out of the main circuit into the displacement chamber 6 or another interior chamber of the pressure medium pump 3; and in the event of a slight excess pressure, the leakage fluid 7 is fed into the main stream or more specifically into the main circuit of the drive system 1.

FIGS. 3 and 4 show schematic circuit diagrams of a hydraulic drive system 1 comprising a hydraulic actuator 2, which is designed as an oscillating motor 23. Oscillating motors, in particular so-called single bladed oscillating motors, are known. They are used, for example, in connection with stabilizers for roll stabilization of a motor vehicle. Such an oscillating motor also has a housing and a rotor, which comprises a shaft and at least one blade. The blade is used to divide a working chamber, enclosed by the housing, into at least two pressure chambers. The two pressure chambers are, as shown, connected in each instance to the one high pressure port 15, 15′ of the pressure medium pump 3. In order to cause the blade to rotate relative to the housing, a chamber is charged with more or less pressure medium 4, so that the result is a defined relative position between the housing and a rotor that is connected in a rotationally rigid manner to the blade.

When the pressure medium pump is in a neutral position, the position of the blade and the rotational position of the rotor are also fixed. The rotor can be rigidly attached to stabilizer rods of a suspension system of a motor vehicle and, as a result, can raise or lower the position of a wheel relative to the top section of the chassis of the vehicle.

In order to be able to implement with low losses such actuating tasks with a drive system 1 that largely dispenses with valves, preference is given to a pulse width modulated electric motor 5 that lends itself well to the drive of the pressure medium pump 3. The actuation of such an electric motor is advantageously performed by a control and/or regulating device (not shown in detail) that processes the sensor signals in the form of measurement values that relate specifically to the trip and the vehicle; and these measurement values are passed on accordingly. In addition, an adaptive control can be realized to the effect that the torque is measured at an output shaft of the electric motor or the oscillating motor and also flows into the actuation of the motor 5. It is also possible to replace the said electric motor with an internal combustion engine of the conventional design, for example, a diesel engine.

The electric motor can also be electronically commutated, so that the result is an improvement in efficiency in addition to an increase in reliability and operational safety. Moreover, such electronically commutated motors are not only less demanding to design, but also less complicated to manufacture. See also in this respect the disclosure of DE 10 2007 058 859 A1.

The exemplary embodiments according to the FIGS. 3 and 4 show in each instance a drive system 1 with a fluid-conducting connection 8 that is provided for discharging the leakage oil 7 out of the displacement chamber or the low pressure chamber of the pressure medium pump in the same way as shown in the FIGS. 1 and 2, specifically, by way of check valves 9, 16, which are connected to the high pressure ports 15, 15′. FIG. 3 shows the drive system 1, which is provided with an additional pressure accumulator 10, which is connected to the respective low pressure port 14 as a function of the operating direction of the pressure medium pump 3, which overall provides for an absolute compensation of the pressure medium losses in the drive system 1 as a whole. The pressure accumulator 10 guarantees a low hydraulic pressure at the point that the leakage fluid is recirculated. At the same time, the drive system 1 is hydraulically prestressed to the extent that a negative pressure and, thus, the cavitation associated with such a negative pressure cannot occur in the drive system 1. In the exemplary embodiment of the drive system 1 shown in FIG. 4, this prestressing function takes on the elasticity of the fluid-conducting connection between the individual components of the drive system 1 itself.

In the exemplary embodiments of a drive system 1 shown in FIGS. 5 and 6, two working cylinders 17 that work in opposite directions are shown as the consumers or as the actuating device 11; and these working cylinders can be used as the actuating cylinder 19 for an active suspension system of a motor vehicle that is not shown in detail. Owing to the cross connected connection of the working chambers 13, 13′ of the cylinders, the pistons 20 of the cylinders 17 can move together in an inactive operating phase of the drive system 1, in which the position of the displacement elements of the pressure medium pump 3 is not fixed; and, hence, the pistons enable an unimpeded movement of the wheels of a motor vehicle in the vertical direction.

In the exemplary embodiment according to FIG. 5, a pressure accumulator 10 is shown in the same way and in order to elucidate the same function as in the exemplary embodiment shown in the FIG. 3.

In the exemplary embodiment of a hydraulic drive system 1 shown in FIG. 7, a valve 18, which is designed as a 2/2-way valve, is connected to each high pressure port 15, 15′ of the pressure medium pump 3. Each of the valves 18 is arranged in a fluid-conducting connection 24 between the high pressure ports 15, 15′ and a pressure medium container in the form of a hydraulic tank. Each fluid-conducting connection 24 has a check valve R, which can be traversed with flow in the direction of the high pressure ports 15, 15′ and blocks in the opposite direction. This switching technique makes it possible to securely clamp a piston 20 of a working cylinder 17 at an end stop point and at the same time enables a controlled movement of the other piston 20 of the other working cylinder 17. In this case, a working chamber of the respective cylinder 17 can be supplied with pressure medium 4 from the pressure medium container (tank). This design measure is necessary, because the required volume of pressure medium cannot be taken from a working chamber 13, 13′ of the piston 20, which in this respect is fixed in position.

The valves 18 can be actuated by the control and/or regulating device. The actuation takes place when the one or the other cylinder 17 demands a rolling moment. A signal for such an actuation of the valve 18 can be obtained through the measurement of a rolling moment at the respective piston 20 or through a pressure signal at a valve 18 or, in addition, from a torque signal of the motor 5.

Just the various check valves alone that are employed are adequate for the function. In practice, they should be check valves with a so-called minimum AP. As an alternative, directional control valves with a very fast activation could be provided. Even combinations of a directional control valve and a check valve (see FIG. 7) are possible and constitute a good solution. For the actuation signal, this signal has to correspond to the direction of the rolling moment; and the magnitude of the rolling moment is immaterial. In summary, it should also be pointed out that the proposed solution provides a very energy-efficient circuit for the hydraulic roll stabilization system. In this respect, only that amount of energy is provided that is actually needed for the roll stabilization. In the previous systems, the pump is connected, as explained, to the drive train, which means that there is always a volumetric flow to be transported, hence a large amount of energy is required. If no roll stabilization is necessary, the volumetric flow is pumped in a circuit at a low pressure, a state that denotes a loss. If roll stabilization is required, then the solutions known from the prior art transport the volumetric flow at a pressure that corresponds to the rolling moment, even though in reality, after the pressure for the rolling moment has been reached, it would only be necessary to compensate for the leakage. The solution according to the invention minimizes such energy losses in an effective way. 

1. A drive system having at least one hydraulic actuator (2), which is supplied with a pressure medium (4) by means of at least one pressure medium pump (3) in a main circuit, wherein the pressure medium pump (3) is a speed-controlled pressure medium pump, which is driven by a motor (5) at a variable speed and a controlled torque, characterized in that the leakage fluid (7), escaping from a displacement chamber (6) of the pressure medium pump (3), is discharged through a fluid-conducting connection (8) having at least one check valve (9, 16), and that the at least one check valve (9, 16) is installed in the fluid-conducting connection (8) in such a way that the check valve prevents the pressure medium from flowing back out of the main circuit into the displacement chamber (6) or another interior chamber of the pressure medium pump (3) and feeds the leakage fluid (7) into the main circuit of the drive system (1) when there is a slight excess pressure.
 2. The drive system according to claim 1, characterized in that the pressure medium pump (3) has at least one low pressure port (14, 14′) and/or at least one high pressure port (15, 15′).
 3. The drive system according to claim 2, characterized in that the leakage fluid (7) is discharged at least at one low pressure port (14, 14′) of the pressure medium pump (3).
 4. The drive system according to claim 2, characterized in that the fluid-conducting connection (8) runs through the respective low pressure port (14, 14′) to at least one high pressure port (15, 15′); and that each port (2) of the connection (8) has a check valve (9, 16).
 5. The drive system according to claim 1, characterized in that the pressure medium pump (3) is an axial piston machine; and that two check valves (9, 16) that open in opposite directions are activated in the fluid-conducting connection (8); and the fluid-conducting connection (8) is connected in parallel to a high pressure port (15, 15′) respectively of the axial piston machine.
 6. The drive system according to claim 1, characterized in that the pressure medium pump (3) is bi-directional in the two-quadrant operating mode or is speed-controlled in the four-quadrant operating mode.
 7. The drive system according to claim 1, characterized in that the pressure medium pump (3) charges a pressure accumulator (10) with a pressure medium (4); and this pressure accumulator also charges the hydraulic actuator (2) with pressure medium (4).
 8. The drive system according to claim 1, characterized in that the hydraulic actuator (2) is an actuating device (11) for a hydraulic system for roll stabilization of a vehicle.
 9. The drive system according to claim 7, characterized in that the pressure accumulator (10) prestresses the pressure medium (4) on the intake side (12) of the pressure medium pump (3).
 10. The drive system according to claim 7, characterized in that the pressure accumulator (10) charges the pressure medium (4) in a working chamber (13, 13′) of the hydraulic actuator (2).
 11. The drive system according to claim 7, characterized in that the hydraulic actuator (2) and the pressure medium container are actuated by a directional control valve in such a way that a suction process is realized.
 12. The drive system according to claim 1, characterized in that the pressure medium pump (3) is used for supplying pressure medium to two hydrostatic working cylinders (17) that work in opposite directions.
 13. The drive system according to claim 12, characterized in that the working cylinders (17) that work in opposite directions are connected in each instance to a valve (18) for draining the pressure medium (4) out of each one of their working chambers (13, 13′).
 14. The drive system according to claim 13, characterized in that a check valve (R) is arranged parallel to the valve (18) in a pressure medium line bridging the valve (18); and that at least one check valve (R) enables the pressure medium (4) to flow out of the pressure medium container to the respective hydraulic actuator (2). 